This may sound noob-ish, but how do I accept a clan war invite?

I dont really know but i suppose you have to go to clans on the top part of the forum and then click on war room, you should see the invitations there and if your rank is enough you can accept it.

Click on "Clans" next to "Market" and from there you can view your clan page. On the left hand size you will see "War Room" Click on that. If you have an invite waiting and you are high enough rank to accept you will have an option to accept it.

Note* You're clan has to have a clan page to clanwar and you will need to be in the top 3 ranks of the clan to accept/send invites for wars.